Semiconductor devices are used in a large number of electronic devices, such as computers, cell phones and others. One of the goals of the semiconductor industry is to continue shrinking the size and increasing the speed of individual devices. Smaller devices can operate at higher speeds since the physical distance between components is smaller. As the size and geometries of semiconductor devices get smaller, however, the voltages that the devices withstand get smaller too.
In the field of submicron CMOS devices, higher speeds and smaller power supply voltages require that the threshold voltage, VT, of the transistor be tightly controlled. First, better control of the transistor threshold voltage allows for more uniform performance of high speed parts, as well as making the circuit design easier because of the more predictable device characteristics. Second, if the threshold voltage is too high, the device may operate slower. If, on the other hand, the threshold is too low, device leakage may be too high. In submicron CMOS devices, however, better VT controllability is desired, but better VT controllability becomes more difficult to achieve as the geometries shrink.